Surface treatment of aluminum or aluminum alloys



'Petente'd July 2i, 1%2

cosmos WTBENT or as 1 1| annoys Martin Tosterud, New Kenslngton, Pa", amiznor to aluminum Company of America, Pittsburgh, Pa, a corporation Pennsylvania 1e Drawing. Applicationiune 2?, 194a, Serial No. 342,707

3 Claims. This invention relates to the treatment of aluminum and aluminum alloy surfaces to produce thereon colored coatings, particularly colored coatings of a permanent light fast nature. The

of the iron oxide in the aluminum oxide coating.

The present invention comprises a method of shooting the precipitation oi" iron oxide in an aluminum oxide coating. By the method of this invention a wide range of colors from light golden yellows to dark reddish browns may be produced. All of these colors areicharacterlzed by permanence, resistance to fading on exposure light, and a pronounced resistance to weathering. The stability of the solutions utilized in the coloring step is very marked and a large number of oxide coated aluminum articles can be colored in a given solution without substantial variation adherent.

' adsorbent nature of the coating. In order to in the color produced. The solutions utilized are capable of adjustment by means of the dition obtain the most satisfactory results the oxide coating should be adapted to adsorb a substanterm aluminum, as used herein and in the 5 tial amount of the solution with which, as above appended claims, includes this metal in every mentioned, it later comes in contact. A further degree of purity. The term aluminum alloys, desirable quality of the oxide coating is its thickas used herein and in the appended claims, inness, a comparatively thick coating being prefercludes all aluminum alloys having an aluminum able. However, the thickness of the coating is content of 50 per cent or more. not a controlling factor, and some comparatively The invention particularly relates to the colorthin coatings, artificially produced, will give good ins of aluminum oxide coatings by the precipltaresults. Being adsorbent, the. coating is necestion of iron oxide therein. While it is apparent sarily porous. that iron oxide has characteristics which make According to the present invention and as the it most desirable as a coloring medium for oxide is first part thereof, the aluminum or aluminum. coated aluminum articles, all previous efiorts alloy article is treated to produce upon its surhave failed in the coloring of oxidecoated eluface an oxide coating which is hard and dense minum with this material. This failure is prob= and which has adequate adsorptive capacity for ably due to the fact that the common hydrolyzretaining the coloring agent. The term "oxide able salts of iron, namely, ferric chloride, ferric so coating," as used herein and in the appendw nitrate, and ferric sulfate in solution give a high. claims, is a well known designation of the art hydrogen ion concentration. Eflorts to adjust which describes slayer of aluminum oxide artithe hydrogen ion concentration oi solutions of ficially produced on the aluminum or aluminum these salts with an alkaline material such as alloy surfaces by treatment of the metal surfaces ammonium hydroxide invariably result in the as with acids, such as sulfuric, chromic, etc., or alprecipitation of'lron hydroxide from the solution. kalies, such as sodium carbonate or the alkali Actually, it has been found that almost all of the sulfates and acid sulfates such as sodium sulfate iron is removed from the solution before the hyor sodium acid sulfate, etc, all with or without drogen ion concentration can be adjusted to a the addition of other substances and some with range which will give an efiective precipitation so and some without the use oi externally-applied electrical energy. The term ends coating" does not include the very thin film of-aluminum oxide which is naturally formed upm that metal or its alloys by reason of contact with the air. 7

The manner in detail of producing omde coat be specifically described. The coatings which have been found to give the best results are the anodic coatings produced in oxalic acid solutions containing from 3 to 10 per cent oxalic acid, anodic. coatings produced in chromic acid solutions containing from 3 to lo percent chremlc of an alkaline material such as ammonium hyk acid, and anodic coatings produced in sulfuric droxide to give a hydrogen ion concentration acid solutions containing from 1 to '10 per cent which will be effective to precipitate iron oxide sulfuric acid. I The preferred anodic coating in the aluminum oxide coating being treated. A treatment is with a solution containing from 10 pronounced advantage of the present invention to 25 per cent sulfuric acid. lies in the fact that the coloring of the oxide The present invention is predicated upon the coating by means of the treatment herein dediscovery that iron oxide can be precipitated in scribed is efiective in a single step operation and the aluminum oxide coatings from aqueous solupennits adsorption by the oxide coating. tions containing ferric oxalate. Suitable aque- In practicing my invention in the preferred one solutions containing ferric oxalate may he manner the aluminum or aluminum alloy which obtained by the use of ferric oxalate itself or is to be treated is first provided with an oxide almost any combination of materials which will coating of certain characteristics. In order that give a ferric oxalate solution. Suitable solutions the coating willnot later separate from the metal may be produced by the addition of soluble oxsurface, itisneeessary that it be hard, dense, and alates to solutions of the common ferric salts I Of great importance, however, is the such as ferric chloride, ferric nitrate, and ferric. sulfate. Soluble oxalates which are particularly suitable as additions to solutions of ferric chloride, ferric nitrate, and ferric sulfate are the soluble alkali metal oxalates including ammonium oxalate. The advantages of the present invention can apparently be obtained independent of the method used for obtaining the solution of ferric oxalate.

While almost any solution containing ferric oxalate will give the advantages of the present invention, a preferred solution is prepared by the addition of a soluble alkali metal oxalate or ammonium oxalate to a solution of a soluble ferric salt in proportions which give about 1 to 3 mols of the soluble-alkali metal or ammonium oxalate to 1 mol of ferricoxalate. Solutions pre-. pared in these proportions can be used through a wide range of temperatures and concentrations to prbduce a wide range of colors, from light yellow to dark reddish, brown, depending in part upon the temperature of the solution and the time of treatment. The hydrogen ion concentration ofthese solutions may be adjusted through a wide range by the addition of a sui able alkaline material such as ammonium hydroxide without any substantial loss of iron from the solution.

These solutions of ferric oxalates and alkali mol of ferric oxalate, and having an adjusted every mol of ferric oxalate. The adjusted pH metal oxalates or ammonium oxalates canbe effectively used in a hydrogen ion concentration range, varying from pH 2 to pH 7. The most satisfactory hydrogen ion concentration for general purposes is from pH 4 to pH 6.5, and for most purposes it is preferred to operate in a range between pH 6 and pH 6.5.

The concentration of these solutions can be varied widely to accommodate other operatin conditions. Very dilute solutions and saturated' solutions may be effectively employed in the process but generally the most effective results can be obtained when theferric oxalate solution or the ferric oxalateealkali metal or ammonium oxalate solution is in a concentration ran e of the ferric salt .of from 3 to 20 per cent with a range of from 5 to 10 per cent preferred.

Solutions of the above concentration may be .used at temperatures ranging fromatmospheric to about the boiling point. .The best results in, the shortest time are easily obtainedwhen the solutions are used at temperaturesran il from 120 F. to 140 F. In general it is preferable to use slightly higher temperatures when 'ferrlce oxalate-alkali metal or ammonium oxalate solu-v tions are used than when using ferric oxalate solutions alone.

' solution containing The method of treating an oxide coated alu-' minum article in the solutions above described consists in merely immersing the article in the solution. The time of immersion in any given solution is dependent upon the color desired. In general, the longer immersion time, the darker the color produced. Suitable examples of com-- mercial procedures of producing oxide coated aluminum articles having ferric oxide precipitated therein are given hereinbelow. These examples are merely typical of the general proce-- dure employed in producing satisfactorily colored oxide coated aluminum articles in accordance with the invention. and it is to be understood that the conditions employed may be varied widely asdescribed hereinabove to obtain the characteristics desired in the product.

vplus 5 to 10 per cent ferric oxalate.

Example 1 Aluminum articles coated as described in Example 1' are washed and immersed in a 10 per cent ferric oxalate solution to which has been added three mols of potassium oxalate for each pH value of from 6 to 6.5 for a period of 20 minutes at F.- The colored articles are then washed, dried, and buffed.

Example 3 Aluminum articles coated as described max ample 'l are washed and then immersed in a 5 per cent ferric oxalate solution to which 1 to 3 mols of potassium oxalate have been added for value of the solution is between 6 and 6.5. The articles are immersed for a period of 20 minutes at temperatures ranging between 122 F. and 140 F. The colored articles are washed, dried, and buffed.

Example 4 Aluminum articles coated as described in Example '1 are'wmhed and immersed in a solution containing from 5 to 10 per cent ferric nitrate potassium oxalate, or a solution containing 5 to 10 per cent ferric sulfate plus 5'to 10 per cent poiassium oxalate, or a from 5 to 10 per cent ferric chloride plus 5 to 10 per cent potassium oxalate, respectively, for a period of 20 minutes at a temperature of 122 F. The colored articles are then washed, dried. and bufied.

It is to be understood that the invention is not .to be limitedto the specific details herein described, since these can be varied without departure from the spirit of the invention as defined in the. appended claims.

I claim:

1. Method of coloring aluminum and alumi num'alloy articles having on their surface an artificially produced oxide coating, comprising immersing the article in a solution containing ferric oxalate.

2. Method ofcoloring aluminum and aluminum alloy articles having on their surface an oxalates and-ammonimn oxalate in an amount equal to about 1 to 3 mols to each mol of the MAR'IIIN TOSTERUD.

and alumi- 

